The plasma transport of Cobalamin (Cbl; Vitamin B.sub.12) to all tissues/cells occurs bound to a plasma transporter, transcobalamin II (TC II), by receptor-mediated endocytosis [1] via transcobalamin II-receptor (TC II-R). Recent studies [2] have shown that TC II-R is expressed as a non-covalent homodimer of molecular mass of 124 kDa in all human [2], rat [3] and rabbit [4] tissue plasma membranes. The plasma membrane expression of TC II-R is important for the tissue/cellular uptake of Cbl since its functional inactivation in vivo by its circulatory antiserum causes intracellular deficiency of Cbl which in turn results in the development of Cbl deficiency of the animal as a whole [4]. Although TC II-R is expressed in the plasma membrane of all cells, its polarity of expression in epithelial cells is not known.
Recent immunoblot studies [3] have revealed that in the rat kidney, TC II-R protein is expressed in both the isolated apical and basolateral membranes with an enrichment in the basolateral membranes by about 8 fold. However, how TC II-Cbl internalized via the TC II-R from either the apical or basolateral surface is processed is not known. Recent TC II-.sup.57 [Co]Cbl uptake studies [4] using filter grown polarized Caco-2 cells have shown that .sup.57 [Co]Cbl taken up from the basolateral side in these cells was utilized as Cbl coenzymes suggesting that these cells derive Cbl essential for their use from the basolateral side.
Despite these studies, the details of intracellular sorting of Cbl and TC II by a polarized epithelial cell are poorly understood. The present studies were undertaken to address the issues related to polarized expression and function of TC II-R in human intestinal derived Caco-2 cells, a well established cell model used extensively to study nutrient transport and general intestinal epithelial cell biology [5, 6].
Needed in the art of cell biology is a method by which cell surface receptors, such as TCII-R, that bind to polypeptide ligands can be used to transport various drugs, including Cbl, across epithelial cell barriers, such as the one that exists in the intestine.